This invention relates to yttrium iron garnet films and to a method for their preparation. More particularly, this invention concerns itself with a method for controlling the resonance frequency of single crystal yttrium iron garnet (YIG) films grown by liquid phase epitaxy (LPE).
Single crystal yttrium iron garnet having the formula Y.sub.3 Fe.sub.5 O.sub.12 and being hereinafter referred to as YIG, is an important crystalline material finding wide application for use in a variety of electronic devices. The material can be utilized in either its single crystal bulk form or as a single crystal film grown on a suitable substrate. In bulk form, the material is often used as an optical modulator, a magnetic flux sensor and in magneto-optical memory devices because of its faraday effect. The major interest in the film form is for applications in magnetic bubble domain devices and in microwave signal processing devices. The high quality of the film form, especially those grown by liquid phase epitaxy techniques (hereinafter referred to as LPE), suggests that thin films, on the order of 1 mm. in thickness, are superior to flux grown bulk crystals in the fabrication of garnet spheres which are used in a number of microwave devices.
In liquid phase epitaxy, the YIG films are grown on a suitable substrate by the isothermal dipping method using a PbO--B.sub.2 O.sub.3 flux. The substrate is held horizontally in the flux and rotated around a vertical axis to produce the thin YIG films. However, it is often required to grow YIG films which have specific resonance frequencies or which have specific differences in resonance frequency in a fixed applied field. Heretofore, it has proven to be extremely difficult to control the resonance frequencies using liquid phase epitaxy. However, with the present invention, it has been found that the incorporation of several percent of lead as a substituent alters the lattice parameter of the YIG film thereby controlling the resonance frequency. The lead need not be added to the flux since it is already present. However, by growing the YIG at sufficiently low temperatures, the lead can be incorporated as a substituent for the Yttrium component of the YIG film in amounts as much as several weight percent.
Although the incorporation of lead as a substituent for YIG films is well known, its use as a means for controlling resonance frequency by utilizing a particular flux component in combination with specifically controlled growth temperatures, has not been previously recognized. The advantages of the present invention are that the flux of LPE melt is simple, the degree of lead incorporation is easily and effectively controlled, and a broad range of lead concentration can be prepared.